Clinker hydraulic binder, use and method for making same

ABSTRACT

A clinker-type hydraulic binder obtained by burning comprising a magnesia spinel mineralogical phase and at least a calcium aluminate mineralogical phase, with a lime content less than 15% of the binder by dry weight. The magnesia spinel represents between 68% and 81% by dry weight of the binder and preferably the calcium aluminates consist essentially of CA and CA 2 , with C═CaO and A═Al 2 O 3 . The invention also concerns the use of such a binder for making a refractory concrete and a method for making such a binder. The invention is useful for making steel ladles ( 1 ), preferably for their wear lining.

The present invention relates to a clinker-type hydraulic binderobtained by burning comprising a magnesia spinel mineralogical phase andat least one calcium aluminate mineralogical phase, with a lime contentof less than 15% of the binder by dry weight. It also relates to the useand a method of making such a binder.

Ladle steel metallurgy has developed these last years up to become a keypoint in the steel-manufacturing process. A ladle is a real chemicalreactor with internal temperatures adapted to reach 1700° C. and beingable to contain up to 300 tonnes of melted material. Conventionally usedrefractory concretes (first shaped, and then more and more monolithic)in steel ladles are not satisfactory any more and the performancesthereof in such a field have to be improved.

In particular, the steel ladles contain wear linings in contact withsteel and slag and more particularly exposed to slag infiltration andcorrosion. Such wear linings should be able to best resist suchaggressions.

More particularly there is an interest for monolithic concretes with lowlime content (lower than 2.5% by dry weight in concrete) so-called LCCconcretes (Low Cement Concrete), and with very low lime content (lowerthan 1% by dry weight in concrete) so-called ULCC concretes (Ultra LowCement Concrete). The low lime content of such concretes is advantageousfor obtaining a high refractoriness required for applications with steelladles.

The Applicant has described in publication UNITECR'97, vol. III, pp.1347-1354 (1997) of N. Blunt, C. Revais and M. Vialle entitled“Additives in calcium aluminate cement containing castables”, a studyover castable monolithic refractory concrete types based on a blend ofaluminous cement and magnesian spinel, particularly with a low limecontent. The magnesian spinel and calcium aluminates contained in thealuminous cement have therein functions of refractory component ethydraulic component.

The concretes being described in such a publication bring outdifficulties to reach a satisfactory rheology and an easyimplementation.

Other solutions have been proposed to make refractory concretes througha clinker based on magnesian spinel and calcium aluminates.

Thus, Patent FR-1,575,633 discloses an aluminous refractory cementproduced from a blend of 30 to 50% dolomite and 50 to 70% calcinedalumina by burning up to clinkerization or melting.

FR-2,043,678 is an addition certificate application attached toFR-1,575,633, which described an aluminous refractory cement based onmagnesian spinel and calcium aluminates obtained from a blend ofdolomite and calcined bauxite or calcined alumina, lime and magnesia,through burning up to clinkerization or melting.

Japanese Patent Application JP-8-198649 is per se relative to acomposition of refractory cement or concrete based on a calciumaluminate material prepared from lime, alumina and magnesia, throughmelting and/or calcination.

The compositions of the three above-mentioned last documents have theinconvenient that they are not sufficiently well adapted for refractoryapplications in steel ladles, particularly for producing LCC or ULCCconcretes able to resist slag infiltration in steel ladles and resultingcorrosion.

The invention relates to a clinker-type hydraulic binder obtained byburning particularly adapted for producing steel ladles and having avery high resistance to slag infiltration and corrosion comparedparticularly to the known binders.

The binder according to the invention allows to produce refractorymonolithic concretes LCC or ULCC based on magnesian spinel, makingpossible an implementation with very satisfactory reactivity (settingtime) and rheology (fluidity, castability).

The invention also relates to the use of such a binder for making arefractory concrete.

It has also as an object a process for making such a binder, makingpossible an easy implementation from currently available raw materialsand advantageously at a low burning temperature (lower than 1800° C.).

Other advantages associated with the binder according to the invention,besides the refractoriness and the resistance to slag infiltration andcorrosion, include the following ones:

cancellation of the 12CaO.7Al₂O₃ (so indicated C₁₂A₇), except possiblyin a strongly underburnt clinker and only on a temporary basis, suchphase being able to bring about stiffening difficulties in concreteformulations;

binder microstructure being advantageous for the milling thereof so asto reach high granular fineness, so improving the corrosion resistance;

very low content in free residual magnesia, i.e. non combined inmagnesian spinel, so as to be able to prevent the generation of cracksdue to free magnesia in brucite during the implementation step for therefractory concrete produced from the binder.

Thus, an object of the invention is a clinker-type hydraulic binderobtained through burning comprising:

a magnesian spinel mineralogical phase, and

at least a calcium aluminate mineralogical phase with a lime content ofless than 15% of the binder by dry weight.

According to the invention, the magnesian spinel comprises between 68%and 81% of the binder by dry weight.

Surprisingly such high proportions of the magnesian spinel allow toobtain the above-mentioned advantages, in particular good corrosionresistance properties.

By contrast, the known clinker-type binders comprising magnesian spineland calcium aluminates and with a lime content of less than 15% havesubstantially lower magnesian spinel contents. In particular,FR-1,575,633 and FR-2,043,678 disclose proportions comprised between 25and 45% magnesian spinel.

JP-8-198649 relates as per se to a binder with a lime content comprisedbetween 15% and 30%, thus inappropriate for making LCC or ULCCconcretes.

The term “clinker-type binder” means not only the proper clinker, thusthe product before milling, but also the clinker being milled.

Such a clinker may be produced either at high temperature (higher than1800°C.) by melting, for example in an electric oven, or advantageouslythrough calcination (sintering) at low temperature (lower than 1800°C.).

Preferably, the binder is used for making concrete to which it givesmagnesian spinel fine particles. The concrete formulation is thenadvantageously supplemented by fine reactive aluminas and by largemagnesia spinel, as well as other granulates.

The high proportion of magnesian spinel in the binder allows to supplythe total fine spinel of the concrete while preventing the problemsencountered with a direct mixture of aluminous cement and magnesianspinel, as in UNITECR'97 above-mentioned. Moreover, the so-made concretemay have a low or very low lime content.

Preferably, the calcium aluminates are all under crystallized form.

More precisely, it is advantageous that the calcium aluminates should beessentially made of CA and CA₂, with CA being CaO and A being Al₂O₃.

Such a binder composition, with a pattern MA—CA—CA₂ (with M═MgO), leadsto such a surprising and advantageously consequence that the presence ofC₁₂A₇ is prevented, such a phase being adapted to lead to a stiffeningcement.

Advantageously the calcium aluminates CA and CA₂ comprise between 19%and 32% of the binder by dry weight. More particularly it is stronglyinteresting the binder should comprise by dry weight of the binder:

71±2% MA (magnesian spinel)

18±2% CA, and

11±2% CA₂.

Such a composition is in thermodynamic equilibrium in the CaO—MgO—Al₂O₃system, so that C₁₂A₇ cannot be present in this combination, exceptpossibly in a strongly underburnt clinker and on a temporary basis.

In an alternative embodiment, the calcium aluminates are present underan amorphous form, particularly under a vitreous form.

Preferably, the binder is quasi-free from free residual MgO at least asit can be observed on X-ray diffraction spectrum for the binder.

Practically the X-ray diffraction technique allows to insure that thefree magnesia is present in a lower proportion than 0.5% of the binderby dry weight.

Thus, the magnesia present in the raw material is almost combined intospinel. During the keramization step of a refractory concrete from thebinder, since the hydraulic concrete dehydration may lead to a highsteam pressure inside the concrete, crack generations due to thehydration of the magnesia into brucite are thus prevented.

By reference to UNITECR'97, the so-obtained concrete may further possessa particularly advantageously microstructure, since it comprises anintergranular matrix (between granulates of big size) formed with muchfiner grains. Such a property is due to the fact that the magnesianspinel in the binder can be easily milled and makes possible to producevery fine grains.

Preferably, the binder has the following chemical composition by dryweight of the binder:

lime CaO  4 to 12% magnesia MgO 19 to 23% alumina Al₂O₃ 69 to 74%.

More specifically, the binder has advantageously the following chemicalcomposition, by dry weight of the binder:

lime CaO  8.4% magnesia MgO 20.4% alumina Al₂O₃ 71.2%.

The binder comprises advantageously a SiO₂ content of less than 0.5% ofthe binder by dry weight.

Preferably, the binder has a Blaine area surface at least equal to 3000cm²/g and advantageously higher than 4000 cm²/g.

This entity is measured according to the NF EN 196-6 standard. Thebinder comprises such characteristic after milling the clinker, thelimit value indicated giving a preferred level of fineness of the grainswhich may be obtained with the binder according to the invention.

Another object of the invention is the use of a binder according to theinvention for producing a refractory concrete.

Preferably, the binder is complemented by magnesian spinel,preferentially of large size, so that the concrete contains between 20%and 30% magnesian spinel by dry weight of the concrete.

Such spinel proportion is particularly advantageously, since it allowsto obtain good resistances, both to corrosion and slag penetration.

More precisely the concrete is produced advantageously by mixing by dryweight of the binder:

between 16 and 27% of the binder,

between 2 and 13% of fine reactive alumina,

between 0 and 19% of large spinel, and

between 52 and 71% of alumina granulates.

In a particularly advantageous embodiment:

18% of the binder,

11% of fine reactive alumina,

11% of large spinel, and

60% of alumina granulates.

These proportions allow to produce a dense concrete with a theoreticalcompactness comprised between 0.25 and 0.40 because grain size lines areused that can suit to Andreasen mathematical model. The above-mentionedcompositions authorize the proportion of 20% to 30% magnesian spinel.

In alternative embodiments, the reactive alumina being mixed with thebinder is substituted by other materials.

The binder according to the invention is advantageously used in themanufacture of steel ladles, preferably for wear linings of such steelladles.

The invention also relates to a process for producing a binder asdefined above. According to the invention, the binder is made throughfrittering by burning of a blend of raw materials comprising dolomite,alumina and magnesia.

This blend, a source of CaO, MgO and Al₂O₃, has the advantage to give avery good sintering behaviour which can be appreciated by the quantityof uncombined magnesia staying after clinkerization.

Advantageously the raw materials have the following characteristicsseparately or in combination:

dolomite is natural: such dolomite leads, upon the decomposition thereofduring the clinkerization, to the formation of very reactive productsand has also the advantage to be economical;

alumina is metallurgical: such alumina has this advantage to be veryreactive;

magnesia is reactive, preferably caustic, and has advantageously a grainsize 100% lower than 100 μm, preferably lower than 40 μm: magnesia finegrain size is particularly interesting, since it favours a totalcombination of magnesia and consequently prevents residual magnesia tobe present.

In two particularly advantageous embodiments, the following patterns arerespectively used, where dolomite, alumina and magnesia are indicated bytheir tradenames:

Dolomite Samin—Alumina Sandy—Magnesia Briquette

Dolomite Samin—Alumina Pechiney—Magnesia MagChem40.

Preferably, before burning, the raw materials are milled up to a grainsize corresponding to a 2% maximum rejection in a sieve of 65 μm.

This co-milling of the raw materials allows to accelerate the chemicalreactions in a solid phase.

Burning is advantageously carried out at a temperature comprised between1400°C. and 1600°C.

Such relatively low burning temperatures are advantageous in theindustrial and economical fields.

Advantageously, the degree of progression for burning is evaluated bymeasuring the free magnesia content by dry weight of the blend, forexample by R-ray diffraction.

Such a content is in fact representative of the clinkerization beingmade.

When the clinker-type binder has been obtained, it is preferably milled.It is then advantageously used for making magnesian spinel basedconcrete.

The present invention will be illustrated and better comprised throughparticular embodiments, but without limiting the invention, referring tothe annexed drawings wherein:

FIG. 1 shows in a longitudinal section a steel ladle manufactured forexample with the binder according to the invention,

FIG. 2 shows an enlarged view of a part of the edges in the steel ladleof FIG. 1,

FIG. 3 shows the comparative grain size lines of the spinel of a binderaccording to the invention and of two common spinels,

FIG. 4 is a picture showing in a 200× enlargement the microstructureafter keramization of a known refractory concrete based on a directblend of aluminous cement (commercially available under reference “S71”)and magnesian spinel (as shown in Publication UNITECR' above-mentioned),

FIG. 5 is a picture showing with a 200× enlargement the microstructureafter keramization of a CMA refractory concrete obtained from a binderaccording to the invention,

FIG. 6 is a top view of a crucible used for corrosion tests,

FIG. 7 is a side view in section through the crucible of FIG. 6,

FIG. 8 represents a type profile through slag degradation of thecrucible of FIG. 6 and 7,

FIG. 9 is a picture of a first crucible after a corrosion test at1500°C. and during 24 hrs, and

FIG. 10 is a picture of a second crucible after a corrosion test at1500°C. and during 24 hrs.

A clinker-type binder, comprising magnesian spinel representing between68% and 81% by dry weight of the binder and calcium aluminates, is usedfor producing a refractory concrete used for manufacturing a steelladle. Such a steel ladle 1 (FIG. 1) of a substantially frustro-conicalshape comprises a bottom 2, a side wall 3 and a cord 4 extending abovethe side wall 3. The steel ladle 1 is used for conveying melted metal,but may be provided with heating means to produce a heating 10 in thebottom 2. Such heating is for example carded out by induction. In analternative embodiment, it is effected through dipping electrodes.

The edges 2, 3 and 4 of the steel ladle 1 comprise three successivelinings 5, 6 and 7, from the inside to the outside of the ladle (FIG.2), respectively a wear lining 5, an insulating lining 6 and a securitylining 7.

Each of the three zones comprising the bottom 2, the side wall 3 and thecord 4 is formed from a distinct refractory concrete adapted for thegiven zone. The wear lining 5 of the side wall and the bottom is made ofconcrete produced from the above-defined binder.

In operation, the steel ladle 1 is used by increasing temperature ofmelted steel 11 up to high values (can reach 1700°C.). Steel 11 in thesteel ladle 1 is contained in a space limited by the bottom 2 and theside wall 3. It forms then above the steel 11 a slag 12 which is limitedlaterally by the cord 4.

Particular exemplary embodiments of the clinker-type binder are furtherdetailed below.

EXAMPLE 1

23.4% by weight of dolomite Samin,

13.8% by weight of magnesia Nedmag, and

63.42% by weight of alumina Pechiney

are used at start (by dry weight of the binder) and the blend is burntduring 5 hrs at 1450°C. The composition of the final product isdetermined by X-ray fluorescence (Table I).

TABLE 1 Composition of the resulting clinker Composition SiO₂ Al₂O₃Fe₂O₃ CaO MgO Percentage 0.1 71.4 0.2 8.6 19.6

The study of the resulting clinker by X-ray diffraction shows that onlythe desired phases are present, namely CA, CA₂ and MA (with C═CaO,A═Al₂O₃ and M═MgO).

The importance of the selection of the raw materials is obvious from thefollowing comparative trials.

A second clinker is made according to the same operating mode as theformer, but with different raw materials: the alumina is substituted forby gibbsite (hydrated alumina) by using the following proportions:

17.2% by weight of dolomite Samin,

10.1% by weight of magnesia Nedmag, and

72.7% by weight of gibbsite,

and the blend is burnt during 5 hrs between 1400° C. and 1600°C.

For both clinkers, the combination ratio of the phases is measured bythe ratio of the surfaces of X-ray diffraction peaks for MgO and MAspinel.

The results are shown in Table II.

TABLE II Comparison of MgO/MA ratios (ratio of surfaces of X-raydiffraction peaks) Raw materials MgO/MA Dolomite/alumina/magnesia 0.06Dolomite/gibbsite/magnesia 0.35

Thus, it has been shown that the combination ratio of magnesia withalumina to form the spinel depends upon the raw materials being used.The selection of these ones is thus quite fundamental.

The spinel obtained in the binder of example 1 with the blend ofdolomite/alumina/magnesia has quite fine grains compared to commerciallyavailable spinels on the market. That is illustrated by a comparison ofparticle diameters between the spinel in the above-mentioned binder andthe spinel sold under the trade name ALCOA AR78 DIN70.

Such a comparison is carried out through a measurement device sold underthe trade name Malvern Mastersizer, (model S) by calling on MIE theorywith “3QHD” presentation, the particularities of which are a particlerefraction index of 1.729, a particle absorption index of 0.1 and arefraction index of 1.33 for the bearing liquid. Thus, three lines from23 to 25 (FIG. 3) are drawn, giving respectively for known spinels(lines 23 and 24) and the spinel of the binder above (line 25),depending on the particle diameter expressed in μm (axis 21), thecumulated percentage of the total volume (axis 22). Consequently, it hasbeen discovered that the spinel in the binder above comprisessubstantially smaller particles that those of known spinels.

EXAMPLE 2

A CMA clinker according to the invention is prepared from 23% dolomite,13.5% magnesia and 63.5% alumina et the blend is burnt during 5 hrs at1450°C. and a CMA clinker is obtained with the following comparison

CaO: 8.4%

MgO: 20.4

Al₂O₃b: 71.2.

The X-ray diffraction diagram for the burnt clinker shows that only thethree expected phases CA, CA₂ and MA are present.

A refractory concrete is made with such a clinker by mixing thefollowing raw materials (Table III):

TABLE III Raw materials for producing concrete Raw materials Mass %Coarse granulates of tabular alumina ALCOA T60(0-7 mm) 61 Granulates ofspinel Haicheng Houyin Magnesite Products 11 MAS 76 (<1 mm) Reactivealumina fines ALCOA CT 3000 SG 10 CMA clinker above 18 Dispersants(mixture of polacrylates-Darvan 7C-and citric 0.1 acid)

These ingredients are mixed with 4.7% of water with respect to theconcrete formulation.

Following properties of such a refractory concrete are measured:

a strong gas evolution of the concrete is observed, which shows that theconcrete is assuming its location correctly without capturing airbubbles, so reducing the refractory porosity and then improving itsresistance to slag corrosion;

concrete stiffening of the concrete takes place after 40 minutes.

Standard conventional mechanical tests and corrosion tests show that theconcrete suits to good behaviour requirements for applications such as awear lining in steel ladles.

It can be also observed that the resulting concrete has a microstructurehaving an intergranular matrix made of very fine grains, particularlywhen compared with a refractory concrete obtained by directly mixingaluminous cement and magnesian spinel (FIG. 4 and 5).

In the following examples, the indications are as follows:

“CMA 72 binder”, a cement containing 72% of MA, 17% of CA and 11% of CA₂by dry weight of the binder (nominal composition) produced according toan industrial process, and

“CMA 80 binder”, a cement containing 80% of MA, 15% of CA and 5% of CA₂by dry weight of the binder obtained experimentally.

Reference is also made to the aluminous cement known under the tradename Secar 71 for comparison.

The following annotations are used thereafter:

Alu Tab: tabular alumina,

Mesh: number of openings per inch on a sieve,

Alu React CT 3000 SG: reactive alumina commercially available undertrade name CT 3000 SG in ALCOA company,

Alu React P 152 SB: reactive alumina commercially available under tradename P 152 SB in PECHINEY company,

HMP: fluidizing agent made of sodium hexametaphosphate,

Chinese spinel: spinel commercially available in Haicheng Houyincompany,

S71: aluminous cement commercially available under trade name Secar 71,

DARVAN 7 S: fluidizing agent.

EXAMPLE 3

Concretes based on CMA 72 and CMA 80 binders the compositions of whichare given in Table IV and a concrete based on S71 binder the compositionof which is given in Table V have been studied.

TABLE IV Composition of CMA-based concrete Concrete compositionReference Content (%) Quantity (g) Alu Tab ¼-8 Mesh 33.0 660 Alu Tab8-14 Mesh 16.0 320 Alu Tab 28-48 Mesh 6.0 120 Alu Tab <48 Mesh 5.0 100Spinel 0.5-1 mm 7.5 150 Spinel 0-0.5 mm 3.5 70 ALU React CT 3000 SG 11.0220 CMA 72 or 80 binder 18.0 360 TOTAL 100.0 2000 WATER 5.3 106 Sodiumhexametaphosphate HMP 0.080 1.60 Boric acid 0.010 0.20

TABLE V Composition of S71-based concrete Concrete composition ReferenceContent (%) Quantity (g) Alu Tab ¼-8 Mesh 33.0 660 Alu Tab 8-14 Mesh16.0 320 Alu Tab 28-48 Mesh 6.0 120 Alu Tab <48 Mesh 5.0 100 Spinel0.5-1 mm 9.0 180 Spinel 0-0.5 mm 4.0 80 Spinel 0-0.09 mm 10.0 200 ALUReact CT 3000 SG 11.0 220 S71 binder 6.0 120 TOTAL 100.0 2000 WATER 5.3106 Sodium hexametaphosphate HMP 0.080 1.60 Boric acid 0.010 0.20

Rheology and workability mentioned in Table VI are obtained.

TABLE VI Rheology and workability of concretes Pouring test ASTM tableWorking Reference Water Vibrating table (mm) (%) time test (%) 0 mn 30mn 0 mn 15 mn mn S71 5.30 240 130 80 30 45 CMA 72 5.30 230 180 65 40 45CMA 80 5.30 225 210 90 60 35

Table ASTM indicates a shock table for standard trials according tostandard ASTM C230.

It has been discovered that concrete fluidities, represented byspreading measurements at different periods, are similar or higher inthe case of CMA 80. Moreover, workabilities represented by the workingtime are identical, except for measurement uncertainties.

With a constant water content, adjuvantation for CMA 72 suits to CMA 80.It is to be noticed that the spinel enrichment of CMA 80 goes with avery significant fluidity gain. With a constant fluidity, the watercontent may be reduced. However, the concrete presents then a fall ofmechanical performances (see example 5).

Such results show that the CMA 72 or 80 based concretes allow for aworking time approximately equivalent to the one provided by the Secar71 based solution.

EXAMPLE 4

Rheology and workability tests are made on the concrete of example 3with CMA 72. The following trials implement two adjuvantations calledrespectively adj1 and adj2 and shown in Table VII. The results obtainedfor rheology and workability are given in Table VIII.

TABLE VII Adjuvantations for concrete of example 3 (with CMA 72)Composition of adjuvantation adj1 adj2 Content Quantity Content QuantityReference (%) (g) (%) (g) Water 5.3 106 5.3 106 Sodium hexametaphosphate0.080 1.600 0.080 1.600 HMP Boric acid 0.010 0.200 0.015 0.300

TABLE VIII Rheology and workability (adj1 and adj2) Pouring test ASTMtable Working Reference Water Vibrating table (mm) (%) time test (%) 0mn 30 mn 0 mn 15 mn mn adj1 5.30 230 180 65 40 45 adj2 5.30 220 185 7030 50

Thus, it has been shown that the HMP content brings about concretedeflocculation and fluidity. Moreover, the boric acid addition does notmake the workability longer significantly. The adj1 adjuvantation givethus a good compromise.

EXAMPLE 5

In such example, the results on the thermo-mechanical properties ofconcretes of example 3 through comparative tests with the concretehaving a reference composition (S71 binder, that does not containmagnesian spinel) are given.

The mechanical performances of the concretes in cold conditions andafter burning are given in Tables IX and X, wherein F and C arerespectively the bending and compression resistances.

TABLEAU IX Mechanical resistances in cold condition and after stovingMechanical resistances (Mpa) After stoving at Reference Water In coldcondition 110° C. test (%) F6h C6h F24h C24h F C S 71 5.30 — — 5.3 57.712.8 100.7 CMA 72 5.30 1.3 13.8  6.6 64.6 10.9 98.9 CMA 80 5.30 0.6 9.64.6 36.9 10.2 83.1

TABLE X Mechanical resistances after a thermal treatment and coldbreaking Mechanical resistances after a thermal treatment Reference Eauand cold breaking (Mpa) test (%) F800° C. C800° C. F1100° C. F1100° C.F1500° C. F1500° C. S 71 5.30 9.8 87.3 13.1 77.6 47.3 >168 CMA 72 5.3010. 110.8 17.4 112.2 37.9 >168 CMA 80 5.30 9.9 105.0 16.4 111.2 32.4>168

It has been discovered that after a thermal treatment at 800° C., themechanical resistances developed by the CMA are higher than those givenby the S71. Moreover, concrete keramization takes place at a lowerburning temperature with CMA binders than with S71 cement.

EXAMPLE 6

In this example, concrete slag corrosion has been studied. To do so,crucibles 30 (FIG. 6 and 7) are made in moulds so that each crucible 30has a cubic shape with a width of I, one top side 32 of which isrecessed with a cavity 31 forming a cylinder of a diameter d and a depthp. Dimensions I, d and p are for example respectively 100, 50 and 50 mm.

Concrete quantity required for making a bloc is 2.5 kg. For each test,two moulds are cast simultaneously. 5 kg of concrete are introduced intothe bowl of Rayneri-type mixer, and then mixing water. Mixing is made atsmall speed. Mixing is then poured into the shaping moulds, then therecessing core is put in place. A vibration of the mixture is carriedout during 1 minute so as to remove air bubbles. For setting, thesamples are arranged in a humid cabinet during 24 hrs at 20° C. Thecubes are then released, dried in a stove at 110°C. during 24 hrs, andthen burnt in a muffle furnace at 1500° C. during 6 hrs so as tokeramize them. As the surface condition of the cube walls is notperfectly smooth, a surface treatment is carried out in the recessbottom with a flat bottom auger. Such surface is a reference formeasuring the corrosion effect.

The corrosion trial consists in putting the slag-containing crucibles 30in a temperature-controlled furnace during 24 hrs. During this period,the slag corrodes the concrete of the crucibles 30.

Before treatment, the depth p of the cavity recess 31 is measuredprecisely on each crucible block through a metal rule. The cavity 31 isthen filled with slag. The block is introduced into a muffle furnace, inan envelope in alumina filled itself with alumina powder to prevent thefurnace deterioration in case where there would be a cracking of thecrucible 30. As specified formerly, a thermal treatment is carried outat 1500° C. or 1600° C., with the following profiles:

profile 1: temperature rise of 100°C./hr up to 1500° C., holding time of24 hrs, and then free fall down to 20° C.;

profile 2: temperature rise of 100°C./hr up to 1600° C., holding time of24 hrs, and then free fall down to 20°C.

After a thermal treatment, the resulting crucibles 35 have a cavity 37(FIG. 8). They are chopped into two parts passing through the cavitycentre 37. Typically, two discrete zones 38 and 39 are distinguished inthe part damaged by the trial:

corrosion, i.e. the block part destroyed by the slag (zone 38), and

impregnation, i.e. the penetration depth of the slag in the concrete(zone 39).

The corrosion is calculated by a depth difference cavity before andafter thermal treatment (cavities 31 and 37). The impregnation isevaluated by measuring the slag penetration in various points of thecrucible 30.

Concretes having the compositions indicated in the Table XI aresubmitted to such a test. The samples are called by names in threeparts: the first one indicates the binder (S71, CMA 72 or CMA 80), thesecond one, alumina (A for ALCOA: CT 3000 SG, and P for PECHINEY: P 152SB) and the third one, spinel (R for reference, H for HARBISON and Chfor Chinese).

TABLE XI Compositions of LCC concretes alumina-spinel used for trialsS71 CMA CMA CMA CMA CMA CMA CMA CMA CMA Raw materials A-R 72-A-R 72-A-H72-A-Ch 72-P-R 72-P-H 72-P-Ch 80-A-R 80-A-H 80-P-Ch Alumina Alu Tab T60¼-8 33 33 33 33 33 33 33 33 33 33 Mesh Granul. Alu Tab T60 8- 16 16 1616 16 16 16 16 16 16 14 Mesh Alu Tab T60 28-48 6 6 6 6 6 6 6 6 6 6 MeshAlu Tab T60 < 48 5 5 5 5 5 5 5 5 5 5 Mesh 0.5-1 mm 9 7.5 7.5 7.5 0-0.5mm 4 3.5 3.5 3.5 Spinel R 78 0-0.09 mm 10 Harbison 14 Mesh 11 11 11Chinese 0-1 mm 11 11 11 Total added spinel 23 11 11 11 11 11 11 11 11 11Alumina CT 3000SG 11 11 11 11 Reactive P 152 SB 11 11 11 11 11 Secar 716 Binder CMA 72 18 18 18 18 18 18 CMA 80 18 18 18 Total 100 100 100 100100 100 100 100 100 100 Water 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3 5.3Adjuvants HMP 0.08 + boric acid 0.01

The corrosion and impregnation thicknesses of the crucibles are reportedin the Table XII. Moreover, it can be seen on pictures the condition ofthe crucibles after corrosion trials for S71-A-R (FIG. 9) et CMA 72-A-R(FIG. 10) at 1500° C. and 24 hrs.

Besides such observations, it is to be noticed that for the trials at1600°C., which create particularly severe conditions, certain crucibleshave visibly lost rapidly their content (identified by a star in theTable XII). The damaged thicknesses are thus smaller. However, at such atemperature, all the crucibles have finally been cracked and lost partof their content, except the crucible CMA 80-A-R. The cracks haveappeared in the locations where the wall thickness is the smallest. At1500°C./24 hrs, the degradation of the crucibles is much moresignificant. The crucibles are not cracked and hold their content.

TABLE XII Corrosion tests (24 hrs) at 1500° C. or 1600° C. 1500° C.1600° C. Impreg- Impreg- Corrosion nation Corrosion nation ObservationsS71-A-R 3 mm 2 mm 5 mm 5 mm Cracked* CMA 3 mm 0 mm 8 mm 10 mm  Verycracked 72-A-R 4 mm 1 mm — — CMA 6 mm 1 mm 72-A-H CMA 5 mm 1 mm 72-A-ChCMA 5 mm 1 mm 5 mm 15 mm  Very cracked 72-P-R CMA 5 mm 0 mm 8 mm 5 mmVery cracked 72-P-Ch CMA 4 mm 8 mm Very cracked 72-P-H CMA 6 mm 2 mm Notcracked 80-A-R CMA 4 mm 5 mm Cracked* 80-P-Ch CMA 7 mm 6 mm Cracked*80-P-H °Rapid cracking → Flowing of slag out of the crucible and a smallinteraction.

At 1500° C., the formulations present all a small degradation, althoughthe thicknesses are slightly weaker for the CMA-containing formula CMA72-A-R. The substitution of the control spinel and/or the reactivealumina CT 3000 SG by other products does not modify the corrosionresistance properties. At 1600° C., these observations stay valid at theobservable level.

Such results show the formulation flexibility for CMA compared to theraw materials. Such modifications do not create significant degradationsfor the properties of the concretes. Moreover, a CMA containing 80% ofspinel also presents advantageous corrosion resistance characteristics.

What is claimed is:
 1. A hydraulic binder obtained through burning, saidbinder comprising: a magnesium spinel mineralogical phase, and at leasta calcium aluminate mineralogical phase with a lime content of less than15% of the binder by dry weight, wherein the magnesium spinel comprisesbetween 68% and 81% of the binder by dry weight, and the calciumaluminates are essentially made of CA and CA₂, with C═CaO and A═Al₂O₃,and comprise between 19% and 32% of the binder by dry weight.
 2. Thebinder according to claim 1, comprising by dry weight of the binder,71±2% of magnesium spinel, 18±2% Ca and 11±2% CA₂.
 3. The binderaccording to claim 1, which is substantially free from free residualMgO, at least as can be observed upon X-ray diffraction spectrum for thebinder.
 4. The binder according to claim 1, having the followingchemical composition by dry weight of the binder: lime CaO  4 to 12%magnesia MgO 19 to 23% alumina Al₂O₃ 69 to 74%.


5. The binder according to claim 1, having the following chemicalcomposition by dry weight of the binder: lime CaO  8.4% magnesia MgO20.4% alumina Al₂O₃ 71.2%.


6. The binder according to claim 1, further comprising a SiO₂ content ofless than 0.5% of the binder by dry weight.
 7. The binder according toclaim 1, having a Blaine area surface at least equal to 3000 cm²/g.
 8. Amethod of using a binder according to claim 1 for producing a refractoryconcrete.
 9. A method of using a binder according to claim 8, whereinsaid binder includes magnesium spinel in an amount between 20% and 30%of magnesium spinel by dry weight of the concrete.
 10. A method of usinga binder according to claim 8, wherein the concrete is produced bymixing by dry weight of the binder: between 16 and 27% of the binder,between 2 and 13% of fine reactive alumina, between 0 and 19% ofmagnesium spinel, and between 52 and 71% of alumina granulates.
 11. Amethod of using a binder according to claim 8, wherein said binder isused in the manufacture of steel ladles for wear linings of such steelladles.
 12. A process for producing a binder according to claim 1,wherein said binder is made through frittering by burning of a blend ofraw materials comprising dolomite, alumina and magnesia.
 13. A processaccording to claim 12, wherein said dolomite is natural.
 14. A processaccording to claim 12, wherein said alumina is metallurgical.
 15. Aprocess according to claim 12, wherein said magnesia is reactive and hasa grain size less than 100 μm.
 16. A process according to claim 12,wherein the raw materials are, before burning, milled up to a grain sizecorresponding to a 2% maximum rejection on a sieve of 65 μm.
 17. Aprocess according to claim 12, wherein said burning is carried out at atemperature comprised between 1400° C. and 1600° C.
 18. A processaccording to claim 12, wherein the degree of progression of said burningis evaluated by measuring the free magnesia content by dry weight of themixture.